X-ray safety inspections have become a standard and widely adopted security measure, especially for monitoring items carried by individuals through critical entry and exit points. In recent years, driven by the growing demands of aviation and public safety, X-ray technology has advanced significantly, becoming one of the most commonly used methods in security screening worldwide. This article provides an overview of the fundamental principles of X-ray inspection techniques and explores the most frequently used X-ray screening methods.
With the rise in global terrorist incidents, public security has gained increased attention from the international community. To combat the growing threat of terrorism, governments around the world have implemented stricter security measures at high-risk locations such as airports, train stations, and ports. These measures focus on detecting dangerous items like explosives, drugs, and other contraband. However, due to the vast variety of explosive materials and their diverse physical forms, there is a continuous need for more advanced and accurate detection technologies that can quickly identify prohibited items.
Currently, international research into detection technologies for explosives and other contraband primarily focuses on X-ray, neutron, electromagnetic, and vapor particle detection methods. Among these, X-ray detection stands out as a mature and widely applied technology. It includes techniques such as X-ray transmission, dual-energy X-ray detection, X-ray scattering, and X-ray computed tomography (CT). These methods rely on extracting key physical properties of the objects being scanned, such as density and effective atomic number, to determine their composition.
In theory, knowing the density and effective atomic number of a substance allows for accurate identification of its type. However, existing security inspection methods still face various limitations. Comparing different techniques and implementing multi-level inspection systems are common strategies to enhance detection accuracy. This article explains the basic working principles of X-ray security inspection and outlines the most commonly used X-ray screening approaches.
**Principle**
X-rays are a form of high-energy electromagnetic radiation produced when high-energy electrons decelerate or when electrons transition between atomic energy levels. Due to their strong penetrating ability, X-rays can pass through luggage, packages, and other objects, making them ideal for use in security inspections.
Once generated, X-rays are directed through a collimator to form a fan-shaped beam that illuminates the object under inspection. As the X-rays interact with the object, some are absorbed, while others are scattered depending on the material's properties. The remaining X-rays reach the detector, where they are converted into electrical signals. These signals are then processed to create a grayscale image, allowing security personnel to identify potential threats within the baggage.
**Commonly Used X-ray Safety Inspection Technologies**
The single-energy X-ray transmission method works by measuring the difference between the incoming and outgoing X-ray intensities. This technique is effective for identifying objects with high effective atomic numbers, such as metals, but struggles to detect hidden contraband behind dense materials.
The dual-energy X-ray method improves upon this by using two different X-ray energies to calculate the ratio of attenuation, which helps determine the effective atomic number of the object. This allows for better differentiation between organic and inorganic materials, enhancing detection accuracy. However, it still faces challenges in distinguishing between allowed and prohibited organic substances.
The X-ray backscattering technique is designed to detect items hidden on the outer layers of baggage or in dark compartments. By analyzing the Compton scattering effect, it offers improved sensitivity for low-Z materials, such as plastic explosives. This method has also been applied to inspect large objects like vehicles, where traditional X-ray methods may fail to detect concealed threats.
Despite its advantages, X-ray backscattering cannot simultaneously provide both effective atomic number and density information, limiting its ability to detect internal or partially hidden explosives.
To address these challenges, combining dual-energy and backscattering technologies offers a more comprehensive approach, providing inspectors with additional data to improve accuracy and reduce false alarms. These integrated systems are now widely used in container, vehicle, baggage, and even human body inspections.
**Conclusion**
While various X-ray detection technologies are currently in use across different security applications, no single system can independently provide both effective atomic number and density information. Each technology has its own strengths, limitations, and specific use cases. Therefore, the integration of multiple technologies and the implementation of multi-level inspection strategies represent a promising direction for the future development of security inspection equipment.
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